var detectBrowser = function(name) {
    if(navigator.userAgent.toLowerCase().indexOf(name) > -1) {
        return true;
    } else {
        return false;
    }
};
var width = parseInt(window.screen.width);
var scale = width/750; 
var userScalable = 'no';
if(detectBrowser("qq/")) userScalable = 'yes';
document.getElementById('viewport').setAttribute(
    'content', 'target-densitydpi=device-dpi,width=750,user-scalable='+userScalable+',initial-scale=' + scale); 